1. Technical Field
The present disclosure relates to control of electrostatic charging effects during fabrication of integrated circuits and, in particular, to a system and method for evaluating and adjusting wafer surface charge, in-situ, during a chemical-mechanical planarization (CMP) operation.
2. Description of the Related Art
CMP is a combination chemical and mechanical polishing technique used in the semiconductor industry to planarize the surface of a semiconductor wafer at various times during an integrated circuit fabrication process. A CMP process typically entails polishing the wafer surface using a rotating polishing pad and a slurry made from various chemicals and abrasive particulates, so that both chemical and physical removal mechanisms contribute to the planarization. Following the polishing, the wafer undergoes a cleaning step in which rotating brushes clean off the slurry. Typically, it is desirable to planarize the wafer surface after completing deposition and patterning of one or more film layers, before proceeding to deposit a next layer of material. If planarization is omitted, uneven surface topography of the un-planarized surface can be transferred to, or accentuated in, subsequent layers. If a film is not sufficiently flat across the entire wafer, it may fail to present a surface that remains in focus during a subsequent lithography step. Such non-uniform topography effects are more likely to occur if materials used in subsequent layers are conformal to the wafer and thus do not evenly fill surface recesses.
CMP selectivity is defined as the ratio of the removal rate of a second film to the removal rate of a first film. A high selectivity, for example 100:1, would remove only 0.5 nm of the first film for every 50 nm of the second film, whereas a poor selectivity, e.g., 1:1, would remove the same amount of each film. CMP selectivity is affected by many factors during processing, including chemical concentrations within the slurry, pressure of the pad, and rotation speed of the pad. As device dimensions decrease below 20 nm, CMP selectivity has a greater effect on device performance.
Historically, control of the selectivity has relied upon characterization of the slurry by the slurry supplier. Slurries used for CMP typically include chemically active abrasives such as silica (SiO2), ceria (CeO2), alumina(Al2O3), titania (TiO2), or zirconia (ZrO2), as well as mechanical abrasives such as diamond and silicon carbide (SiC). It is understood that slurry particle size can affect wafer charging by affecting interfacial chemical reactions. Accordingly, slurry manufacturers have learned to control the surface functionality of CMP slurry by adjusting the sizes and spatial relationships of slurry nanoparticles. Such adjustments can be made, for example, by synthesizing fumed, precipitated, or amorphous forms of the slurry compounds, each of which can have a different charging effect on wafers. The ionic composition of slurries can also affect wafer charging, such as, for example, the presence of Ce3+ and Ce4+ ions in a ceria-based slurry. In addition to the ionic composition of the slurry, pH can also affect wafer charging. Slurries can further include chemical additives such as corrosion inhibitors, chelators, biocides, colloidal stabilizers, rate accelerators, oxidizers, surfactants, passivating agents, and dispersible polymers. For example, colloidal silica-based slurries are preferred for use in copper CMP, while fumed silica is preferred for oxide or tungsten CMP.
Once the slurry is evaluated and the selectivity of the slurry chemistry is determined, process engineers hope to find that the CMP selectivity is uniform across the entire wafer. However, in practice, local irregularities can modify the selectivity, causing hot spots where non-uniform planarization occurs. Such hot spots can be very costly if, for example, they were to cause too much metal to be removed at one of the last layers in the process, thus causing product to be scrapped after the maximum investment in manufacturing has already been made. One such local irregularity is electrostatic charge concentrated in a particular location on the surface of the wafer. Surface charge tends to accumulate due to friction during CMP and during the subsequent brush cleaning step, especially when planarizing metal layers. Surface charging can further be affected by the pH of the slurry such that the surface becomes, for example, increasingly more negative as the pH increases. In addition, the slurry particle size can affect the type of charging that occurs. Surface charge can also be imparted to the wafer by electrostatic chucks that hold the wafer in place during plasma processing at deposition or etching operations upstream of the CMP step. A general trend has been observed that after CMP, the surface potential becomes more negative. Although this is the case for all types of films, the surface charging is more obvious on insulator films than metal films. Furthermore, if a particular location on the wafer is charged prior to CMP, the charge is maintained and increased after CMP.
The connection between charging and selectivity is based on experimental observations of the film removal rate changing in response to the charge on the slurry particles relative to the wafer charge. This is particularly true for silica-based and ceria-based slurries. If the slurry charge is opposite that of the wafer, such that charge attraction occurs, there is more opportunity for interaction of the chemical reactants, which increases the reaction rate. The increase in chemical reaction rate results in a higher film removal rate during CMP. In areas where the charge polarity of the wafer matches that of the slurry, charge repulsion causes a reduction in the removal rate. Thus, the ability to control or to neutralize charging allows control of CMP selectivity, both globally and locally.